The present invention relates to a starting control apparatus for an AC load, More particularly, it relates to a starting control apparatus for starting an inductive load such as an induction motor.
In general, a line start system, directly turned on by operation of an electromagnetic switch, has been employed as a starting system for an AC load such as an induction motor because of the low cost.
FIG. 1 is a diagram showing the starting of the three-phase induction motor in the direct system. in FIG. 1, the three-phase induction-motor 1 is connected through an electromagnetic switch 2 to the three-phase AC power source (not shown). The power voltage is applied to the induction motor 1 by turning on the electromagnetic switch 2 to rotate the motor. The counter electromotive force during starting from a static state is zero in the induction motor 1 whereby the starting current is remarkably large. Usually the starting current is about 5 to 7 times the rated current in the rated speed and rated load.
The starting current is continuously fed during the predetermined starting period given by the moment of inertia of the load and the induction motor 1. When the induction motor 1 reaches its rated speed after starting, the power-factor is improved and reaches about 80% at the rated load. However, the power-factor at the static state is about 20 to 30% and accordingly, sometimes, rush current having about 1.4 times to about 5 to 7 times of the rated current is fed by the transient pehnomenon at some phase and turns on the electromagnetic switch 2. The phenomenon is maximum by turning on the electromagnetic switch at zero of the power voltage phase.
The rush current and the starting current are not so large when the capacity of the motor 1 is small. However, when the capacity of the motor 1 is large, the rush current and the starting current are large and the starting time is long whereby it is disadvantageous for the electric line and the motor 1. Accordingly, the line start system can be used for starting a motor having a small capacity but there is a problem is using the direct turn-on system for starting a motor having a large capacity.
The electromagnetic switch 2 has a movable contactor and the switching operation is performed by the attractive force of an exciting coil (not shown) and the repulsive force of a coil spring (not shown). Accordingly, an arc is generated between the contacts during the switching operation whereby abrasion of the contact is disadvantageously caused. The switching operation is performed by mechanical force with the result that noise and vibration are considerable.
It is usual to employ a reactor starting system or Y-.DELTA. starting system for starting a motor 1 having a large capacity. FIG. 2 is a diagram of a circuit for starting a three-phase induction motor by a conventional reactor starting system.
In FIG. 2, the three-phase induction motor 1 is connected through a series connection of a first electromagnetic switch 2a and a reactor 3 and second electromagnetic switches 2b to the three-phase power source (not shown). At the starting of the motor, the first electromagnetic switch 2a is turned on in the OFF state of the second electromagnetic switch 2b. A voltage several tens % lower than the power voltage is applied to the motor 1 by a reactor 3, whereby the rush current and the starting current are suppressed to start the rotation of the motor 1. When the motor 1 reaches a certain speed, that is a certain counter electromotive force of the motor 1 is generated, the first electromagnetic switch 2a is turned off and the second electromagnetic switch 2b is turned on whereby the power voltage is applied to the motor 1 and the speed is increased to reach the rated speed to complete the starting operation.
In the reactor starting system, the rush current and the starting current are advantageously suppressed by applying a partial voltage to the motor by using the reactor 3 at the starting operation, whereby the electric line is protected and the mechanical shock applied to the motor 1 is weakened and a force is not suddenly applied to the load of the motor 1. However, the reactor starting system gives a stepping starting operation whereby a smooth starting operation is not attained. Moreover, two electromagnetic switches 2a, 2b are used, whereby the abrasion of contacts of the electromagnetic switches 2a, 2b, and the generation of noise and vibration are still caused.